We will be studying the organization and evolution of multigene families whose expression is both temporally and spatially regulated within an otherwise quite similar population of cells. These cells form an epithelium surrounding oocytes of silkmoths, and during their terminal state of differention synthesize and secrete more than one hundred distinct chorion proteins. Sequence analysis of both proteins and encoding nucleic acids have shown that chorion genes consist of a small number of gene families. Genetic analysis has shown that these families are clustered on the chromosome. Chorion genes are expressed at different times during development when measured at the level of specific RNA accumulation and protein synthesis. The group of chorion genes (ca. 10% of total) which are expressed during the very late period of choriogenesis encode components which assemble into morphologically distinct surface structures called aeropyle crowns. In the primary species of interest, aeropyle crown formation and synthesis of its components are topographically restricted to a region of the chorion's surface plus overlying cells. We are interested in the level at which this cellular differentiation is controlled. We will compare regions with and without aeropyle crowns in terms of their developmental patterns of specific RNA synthesis and accumulation, and their gene organization. We will also look for possible gene rearrangements or changes in gene copy number. In addition, the organization of very late period genes will be compared with the organization of chorion genes expressed at earlier times. This work will be made possible by the use of libraries of chorion-specific cDNA and chromosomal clones. The distribution of aeropyle crowns is dramatically different in closely related species, and this may result from evolutionary changes in the abundance or arrangement of very late period genes. These hypotheses will be tested using libraries of chromosomal clones available in these related species. The proposed work may shed light on the expression and organization of multigene families in a eukaryotic system, and suggest how evolutionary changes in them may have led to morphological variation.